Setup Failure Codes/PHP DateTime class
The page identifies a minor configuration problem. Some functionality may be impacted but most parts of DAViCal should work fine.
DAViCal does stuff with calendars. That involves doing stuff with dates. Strangely enough, this involves using the PHP DateTime class.
Actually we only really get to depend on this stuff when we start doing really complicated date manipulation, like calculating the instances of a repeating event, so i you know you have simple calendars you may be able to get away without this.
If you're using old CentOS versions you're pretty much stuck with having simple calendars, because they don't have these libraries available, which is probably what got you arriving at this page. There's not much I can really do about that. Sorry.